1. Field of the Invention
The present invention relates generally to a rotary piston engine and, more particularly, to a rotary piston engine provided with means for positively removing residual or burnt inert gases from a plug hole in which electrodes of an ignition plug are disposed thereby to effectively scavenge the plug hole and the plug electrodes.
2. Description of Prior Art
A rotary piston engine is known to have a rotor housing comprising a peripheral housing part and a pair of axially spaced side housing parts secured to opposite ends of the peripheral housing part. The housing parts define a cavity therein. The inner peripheral surface of the peripheral housing part is formed of epitrochoidal curves. A generally triangular rotary piston or rotor is received in the cavity for planetary rotary motion therein. The rotor is cooperative with the inner surfaces of the rotor housing to define therebetween a plurality of working chambers each of which is moved with one of the sides of the rotor and along the inner peripheral surface of the rotor housing and sequentially transformed into intake, compression, combustion expansion and exhaust chambers as the planetary rotary motion proceeeds. An ignition plug or plugs is mounted usually on the peripheral housing part and has a set of electrodes disposed in a plug hole formed in the peripheral housing part and opened in the inner peripheral surface of the peripheral housing part. The plug hole has been designed to have a diameter which is small enough to minimize the flow or leakage of a working fluid caused by the pressure difference between each adjacent pair of working chambers. The leakage unavoidably takes place from one chamber to another across the rotor apex therebetween and through the plug hole when the rotor apex is positioned just in front of the plug hole. Due to the small diameter of the opening of the plug hole, the prior art rotary piston engine has suffered from incomplete scavenging of the plug hole and the plug electrodes therein. Considered from a static point of view, residual or burnt inert gases produced in the plug hole are exhausted and discharged from the plug hole on expansion and exhaust strokes to an extent where the pressure within the plug hole is reduced to substantially the atmospheric pressure. In the succeeding intake stroke, a charge of air-fuel mixture is compressed in the intake chamber correspoindingly to the compression ratio of the engine. The part of the residual gases retained in the plug hole is thus also subjected to compression thereby to form a mass of compressed residual gases. In the event where the mass of compressed residual gases is placed around or adjacent to the plug electrodes, there has been caused a misfire. In addition, even if a volume of ignitable air-fuel mixture is ignited in the plug hole, the presence of the mass of compressed residual gases in the plug hole adversely affects the temperature rise of the combustion of the air-fuel mixture in the plug hole because of the heat capacity of the mass of compressed residual gases therein. Moreover, since the burning velocity in the entire intake chamber is small especially when the engine is operated in a light or partial load condition, the engine is arranged such that the spark discharge is produced before the rotor reaches the top dead center. This ignition timing setting increases the ratio of the mass of residual gases in the plug hole relative to the volume of ignitable air-fuel mixture therein and, thus, increases the frequency in the production of misfires with resultant decrease in the operability of the vehicle on which the engine is mounted and also with resultant increase in the emission of harmful components of engine exhaust gases.